staging: comedi: pcmmio: return error if ao conversion times out

Rename wait_dac_ready() so it has namespace associated with the
driver.

Fix the function so it returns an errno if the conversion times
out. Propogate this errno if it happens.

Define the analog output status register to remove the magic numbers.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>